In communications systems, multipath is a well-understood phenomenon. Signals propagate from a transmitter to varying degrees in all directions. While one image of the signal (known in the art as the direct-path image) travels in a straight line from the transmitter to the receiver, other images are reflected from any number of surfaces and arrive at the receiver at other times, those times being largely dependent on the length of the path traveled from the transmitter to the receiver.
Aside from well-understood issues of fading and destructive interference, multi-path propagation is generally welcomed in the communications arena. In most cases, the direct-path image must penetrate several obstacles before arriving at the receiver. Given a large number of images of the signal following other paths between the transmitter and receiver, it is likely that many of them will penetrate fewer obstacles and experience less attenuation than the direct-path image. Since all images of the signal carry the same information, it is unimportant which particular image is detected and decoded. Thus, the arrival of stronger images that arrive at the receiver by other paths is generally beneficial.
Unfortunately, however, the receiver receiving multipath images of a signal from various paths is not beneficial for location systems. While signals in location systems generally carry some information modulated onto them by the transmitter, the primary purpose of detection of these signals is to gain physical information that indicates the spatial relationship between the receiver and the transmitter. Signal strength or received signal strength information (“RSSI”) systems estimate the distance from a transmitter to a receiver based on the strength of the arriving signal, assuming that the signal is attenuated roughly 6 db each time the distance doubles. Angle-of-arrival (“AOA”) systems estimate the direction of the transmitter from the receiver based on the angle from which the detected signal arrived. Timing-based systems, such as ranging or time difference of arrival (“TDOA”) systems, estimate distances in a system based on the arrival time of signals at receivers, using the approximation that the signals propagate at a constant speed. In all of these cases, detection of an image of the signal other than the direct path image is disruptive, since these other images will generally have different amplitudes, come from different directions, and arrive at different times than the direct-path image.
Because of these considerations, a location system must be capable of detecting and characterizing the direct-path image despite interference from other images of the same transmitted signal. Rejecting images other than the direct path image cannot be accomplished by simple filtering or averaging, since the interfering image is, unlike noise or other emissions, highly correlated with the desired image. For this reason, many location systems use broadband spread-spectrum techniques. Two images of a signal appear as distinct entities if their arrival times differ by a factor on the order of the inverse of the occupied bandwidth. It is generally valid to assume that the image arriving earliest is the direct-path image, since the overwhelming majority of the propagation of any signal is through air on the Earth's surface, and the propagation speed of the signal is substantially equal to the speed of light in free space.
While using broadband modulation addresses the issue of distinguishing the direct-path image from other images arriving along substantially different paths, it assumes that the direct-path signal is strong enough to be detectable. As mentioned previously, the direct-path signal is usually not the strongest, as it must generally penetrate multiple barriers, such as walls, floors, vehicles, or buildings before reaching the receiver. This issue is compounded by the fact that, while a single connection between a transmitter and a receiver is adequate for communication, most location systems require characterization of the propagation paths between the transmitter and several receivers, increasing the required system density significantly. It is well known in the art that, by significantly reducing the information capacity of a system relative to its bandwidth (e.g., by sending largely redundant information), it is possible to increase the effective gain of the system. This increase is called coding gain. Direct sequence spread spectrum (“DSSS”) is one implementation of this technique, and is used extensively by modem broadband location systems, including the Global Positioning System, (“GPS”). A pseudonoise (PN) code is superimposed on the data, generally at a higher modulation rate than the data itself. The receiver is given a priori knowledge of the PN code. The rate at which symbols are modulated onto a carrier is called the chipping rate, and determines the bandwidth of the signal. The rate at which information is transmitted is called the bit rate, and determines the utilized bandwidth. The ratio is the number of chips per bit, and establishes the coding gain for a particular bandwidth. The coding gain may be approximated as 3 db*log2(N), where N is the number of chips per bit. Coherent averaging, which takes into account signal phase as well as amplitude, is possible over the number of chips in a bit because the receiver knows the pattern of the chips within a bit a priori. Because the receiver must know this pattern, it cannot contain any information; in information theory, information is by definition unknown to the receiver, and any data in a message (such as the PN code) that is known to the receiver before the message is detected is not considered information. This is why the tradeoff between coding gain and information rate becomes possible.
The teaching of prior art communication systems design is that coherent averaging to improve the detection of a digital signal can be performed only over a length of time corresponding to the duration of one information symbol or less. A symbol is a quantum of information, which corresponds to one bit in the binary system. The reason for this is that the symbol carries information and, as such, its state cannot be determined a priori by a receiver. If the receiver were to have a priori knowledge of the state of a symbol, information theory holds that no information would be contained therein.
This technique, however, is not without its disadvantages. Most location systems send at least some data with location transmissions. Examples of data needed to supplement location transmissions includes transmitter identification, ephemeris or status data, alerts, configuration data, and control information. Given a particular amount of data to be sent in a location transmission, for each 3 db increase in bandwidth, the duration of the transmission must be doubled. This reduces system capacity and battery life proportionally. Some implementations add a known synchronization pattern to the message in addition to the transmitted data to improve the detection sensitivity. This could be regarded as the sending of two distinct signals, one for location characterization and one for data. It is obvious that this also increases power consumption and decreases system capacity.
Thus, there exists a need for a technique that is capable of utilizing the entirety of the transmitted signal to improve the sensitivity of the receiver to the direct-path image of the signal, without imposing excessive limitations on the amount of information that may be sent with the message.